This invention relates in general to refractory compositions and, more particularly, to a specific refractory composition paticularily suitable for use in contact with molten aluminum alloys.
Refractory materials that come into direct contact with molten aluminum alloys, such as those used in aluminum melting furnaces, remelting furnaces, ladles, troughs, etc. are subject to disruptive attack, penetration, and adherence by various alloying elements, and by dross formed on the surface of the melt.
Historically, refractories used for these applications, mostly fired brick and phosphate bonded moldable refractories, were found to hold up to molten alloy for reasonable periods of time because the operating temperatures were kept relatively low and the alloys used were relatively mild. The refractory grains and binder used in these products were generally based on refractory materials designed for other industries, however, and these materials are not intrinsically resistant to reaction under aluminum melting and holding conditions. Procedures in the aluminum industry are changing, with increased emphasis on throughput rates and more severe alloys, with the result that the older refractories are becoming borderline in their acceptability.
There have been attempts made from time to time to improve the resistance of refractories to attack by molten aluminum alloys by the use of additive materials. McDonald, for example, in U.S. Pat. No. 2,997,402 describes the production of a glassy frit containing 15 percent to 80 percent boron oxide, 5 percent to 50 percent calcium oxide, and 2 percent to 60 percent aluminum oxide, and the production of a fired shape by blending the described frit with an aggregate, pressing and firing. Rubin, et al., in U.S. Pat. No. 3,471,306 improved upon this scheme by using similar ranges of ingredients, but forming the protective glassy frit in situ by virtue of the ingredients being in a reactive form. Both the McDonald and Rubin patents describe the production of bricks that are bonded by a glassy material. The implications of a continuous glassy phase thorugh a refractory are reduced refractoriness and decreased thermal shock resistance.
La Bar, in U.S. Pat. Nos. 4,088,502 and 4,158,568, describes the use of zinc borosilicate frit as an additive in calcium aluminate cement bonded silica and alumina based refractories to render them resistant to aluminum alloy attack. Maczura et al., in U.S. Pat. No. 4,246,035 uses this same additive, with the further addition of boric acid, in a high purity mortar to achieve resistance. Hines et al., in U.S. Pat. No. 4,348,236, discloses the use of calcium aluminate together with borosilicate frit, in particular a borosilicate frit containing 5-14 25% by weight fluorine, as an additive to an alumina refractory to improve aluminum resistance. While the use of zinc borosilicate frit is effective, the cost of commercially available material is higher by an order of magnitude than the additive of the invention described hereinafter, and it refractoriness is lower. Additionally, the presence of a glassy phase in a refractory body renders it more susceptible to thermal shock.
In U.S. Pat. No. 4,126,474, Talley, et al., described the use of BaSO.sub.4 as an additive to render various types of refractory resistant to molten aluminum attack. A potential drawback to the use of this additive material is that, since most barium salts are toxic, care must be exercised to establish that conditions are not present in which the non-toxic BaSO.sub.4 will decompose to form soluble barium compounds, or, if they are formed, that the refractory is subjected to hazardous waste disposal procedures after use.
Henry, in U.S. Pat. No. 3,261,699, and Drouzy, in U.S. Pat. No. 4,174,972, disclose a refractory material suitable for use as a lining material for furnaces and electrolytic cells used for the production of a aluminum wherein an additive material comprising at least one fluoride from the group consisting of alkali metal fluorides alkaline earth metal fluorides, aluminum fluoride and mixtures thereof is added to the refractory. However, fluorine compounds retard cement set times in castable compositions and effect the workability and shelf life of moldables. For these reasons calcium fluoride cannot be added directly to a refractory mixture to import improved aluminum resistance to a refractory mixture without also importing detrimental characteristics to the resultant mixture.
Felice et al., in U.S. Pat. No. 4,510,253, describes a refractory composition for use in contact with molten aluminum alloys containing alumina silica ceramic fiber, refractory binder, and an additive for importing aluminum resistance containing essentially crystalline 9Al.sub.2 O.sub.3 -2B.sub.2 O.sub.3. In U.S. Pat. No. 4,522,926, Felice discloses another refractory composition for use in contact with molten aluminum alloys containing a refractory aggregate, a binder, and 9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3, with the aluminum borate preferrably being a by-product aluminum boron slag from the production of ferro-boron alloys.
The ferro-boron slag utilized by Felice (U.S. Pat. No. 4,522,926) contains relatively large quantities of iron oxide, magnesia and calcia impurities which decrease the refractoriness of products which contain this additive. The slag also contains fluctuating levels of boron and fluorine compounds which retard set times and destroy hydraulic bonding. These problems could be alleviated with a synthetic aluminum borate, however, at addition levels of 5-14 10 percent, the aluminum borate would be detrimental to the physical properties of a refractory and to the corresponding cost of a product containing that percentage of a synthetic additive.